Lasers are used for many applications. In one example, lasers are used in steppers for selectively exposing photoresist in a semiconductor wafer fabrication process. In such fabrication processes, the optics in the stepper are designed for a particular wavelength of the laser. The laser wavelength may drift over time and, thus, a feedback network is typically employed to detect the wavelength of the laser and correct the wavelength as necessary.
In one type of feedback network used to detect and adjust the wavelength of a laser, an etalon receives a portion of the emitted light from the laser. The etalon creates an interference pattern having concentric bands of dark and light levels due to destructive and constructive interference by the laser light. The concentric bands surround a center bright portion. The position of the bright center portion of the interference pattern is used to determine wavelength to a relatively coarse degree, such as to within 5 picometers (pm). The diameter of a light band is used to determine the wavelength of the laser to a fine degree, such as to within 0.01-0.03 pm. The width of a light band is used to determine the spectral width of the laser output. The interference pattern is usually referred to as a fringe pattern. The fringe pattern may be optically detected by a sensitive photodetector array.
Various methods are well known for wavelength tuning of lasers. Typically the tuning takes place in a device referred to as a line narrowing package or line narrowing module. A typical technique used for line narrowing and tuning of excimer lasers is to provide a window at the back of the discharge cavity through which a portion of the laser beam passes into the line narrowing package. There, the portion of the beam is expanded and directed to a grating which reflects a narrow selected portion of the laser's broader spectrum back into the discharge chamber where it is amplified. The laser is typically tuned by changing the angle at which the beam illuminates the grating. This may be done by adjusting the position of the grating or providing a mirror adjustment in the beam path. The adjustment of the grating position or the mirror position may be made by a mechanism which we will refer to as a laser wavelength adjustment mechanism.
FIG. 1 is a graph 10 which illustrates the wavelength drift over a burst of pulses from a laser. In particular, FIG. 1 indicates wavelength shift (i.e., wavelength drift from a desired wavelength output) of a maximum magnitude greater than about 0.1 pm. The circled section 12 of graph 10 indicates a large wavelength shift referred to as a wavelength chirp from the laser. As shown in FIG. 1, after the wavelength chirp, the wavelength output settles down to a wavelength shift of a maximum magnitude of less than about 0.05 pm. Conventional wavelength correction techniques do not adequately correct the large and sudden wavelength shifts in the wavelength chirp.
Excimer lasers operating in a burst mode also produce a pulse energy chirp similar to the wavelength chirp. Prior art methods have been disclosed to minimize the pulse energy chirp. One such method is described in an article by the inventors' co-workers, "Advanced Krypton Fluoride Excimer Laser for Microlithography, SPIE Vol. 1674, "Optical/Laser Microlithography V, (1992) 473-484, see page 480.
What is needed is a technique to correct the wavelength shift in the wavelength chirp of a laser.